ROTATION OF AROMATIC SOLUTES IN SUPERCRITICAL CO2 - ARE ROTATION TIMES ANOMALOUSLY SLOW IN THE NEAR-CRITICAL REGIME

Picosecond fluorescence anisotropy decay measurements are used to exam ine the rotational dynamics of three solutes, 1,3,6,8-tetraphenylpyren e (''TPP''), 9,10-bis(phenylethynyl)anthracene (''PEA''), and N,N'-bis -(2,5-di-tert-butylphenyl)-3 ,4,9,1--perylenedicarboximide (''BTBP'') in supercritical CO2 (35 degrees C = T-c + 4 degrees C) and in a varie ty common liquid solvents. In liquids the rotation times of all three probes show an approximate proportionality to solvent viscosity, in ro ugh agreement with simple hydrodynamic theories. In supercritical CO2 two of the probes, TPP and BTBP, are found to exhibit rotation times c onsistent with the extrapolation of the hydrodynamic trends found in l iquid solvents. In the case of BTBP, these results disagree with recen tly published reports of very long rotation times near the critical po int [Heitz and Bright J. Phys. Chem. 1996, 100, 6889]. However, the ro tation times of PEA deviate significantly from hydrodynamic prediction s based on the viscosity of the supercritical fluid for near critical densities. In this case, it appears that local density augmentation le ads to increased rotational friction on the solute compared to what wo uld be expected on the basis of the bulk solvent properties. Using the observed rotation times, an effective density that is 50-100% greater than the bulk density is estimated for reduced densities (rho/rho(c)) of 0.8-1. Similar estimates of the extent of local density augmentati on are also obtained from the behavior of the electronic frequency shi fts of this solute.